Conventional polyimide resins for liquid crystal alignment films are prepared from the polycondensation of aromatic acid dianhydrides, such as pyromellitic acid dianhydride (PMDA) and biphthalic acid dianhydride (BPDA), and aromatic diamines, such as p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), 4,4-methylenedianiline (MDA), 2,2-bisaminophenylhexafluoropropane (HFDA), m-bisaminophenoxydiphenylsulfone (m-BAPS), p-bisaminophenoxydiphenylsulfone (p-BAPS), 4,4-bisaminophenoxyphenylpropane (BAPP), 4,4-bisaminophenoxyphenylhexafluoropropane (HF-BAPP) and the like.
Polyimide liquid crystal alignment films produced from the conventional aromatic acid dianhydrides and aromatic diamines are excellent in thermal stability, chemical resistance and mechanical properties, but are poor in electrooptical properties, as well as in transparency and solubility, due to the formation of a charge transfer complex. In efforts to solve these problems, Japanese Patent Laid-open No. Hei 11-84319 discloses a polyimide-based liquid crystal alignment film into which an alicyclic acid dianhydride monomer or an alicyclic diamine is introduced, and Japanese Patent Laid-open No. Hei 06-136122 discloses a polyimide-based liquid crystal alignment film having an increased pretilt angle of a liquid crystal and improved stability produced by using a functional diamine having a side chain or a functional acid dianhydride having a side chain.
According to a study undertaken by the present inventors, since the conventional polyimide-based alignment films use polyimides unsuitable for surface tension and polarity to obtain a high pretilt angle of liquid crystals, the spreadability of liquid crystals upon injection of the liquid crystals is poor, causing many defects in the final liquid crystal alignment film, and the controllable range of the pretilt angle is not increased to a satisfactory level.
As demand for liquid crystal display devices has recently increased, there is a continuing need for high quality display devices. Further, as technologies for large-scaled liquid crystal display devices have made remarkable progress, alignment films having a high productivity are more and more required. Thus, there is an urgent need in the art to develop a liquid crystal alignment film which facilitates the pretilt angle control of a liquid crystal over a broad range of angles and has few defects when applied to LCD fabrication processes and shows excellent electrooptical properties, high reliability and superior spreadability of a liquid crystal.